The Netherlands

Few details are known of St Leger's career before 1607, when he killed a man in a duel and took refuge from the law with Hugh O'Neill, Earl of Tyrone, one of the leaders of the rebellion in which his father died. When O'Neill and his companions went into exile a few months later, St Leger accompanied them to Brussels in the Spanish Netherlands but as a Protestant refused to follow them into Spanish service.^[3] Although the 1609 Twelve Years' Truce had temporarily halted the Eighty Years War, he moved to the Dutch Republic and took up a military career. In May 1610, he was pardoned by for the murder by James I^[1] and appointed captain in Lord Cecil's regiment, an English unit in Dutch service that fought in the War of the Jülich Succession, often viewed as the precursor of the Thirty Years War.^[4]

For many English officers in the Dutch States Army, local connections were seen as essential to secure promotion,^[5] which may have been a factor in St Leger's marriage to Gertrude de Vries in 1616. Shortly after, he became part of the patronage network around George Villiers, 1st Duke of Buckingham, whose personal relationship with James I made him extremely powerful. With Buckingham's support, St Leger was knighted in 1618 and awarded estates in Leinster, part of a policy of transferring lands from the Catholic Irish to Protestant settlers.^[3] After the truce ended in 1622, St Leger served under Sir Charles Morgan in the defence of Bergen op Zoom.^[4] In 1624, he was appointed agent for lands in Upper Ossory granted to Buckingham by James I.

In October 1625 he commanded a regiment during a failed attack on Cádiz; most of the landing force became drunk and St Leger claimed he "had never seen men acting with such beastliness".^[4] Although Buckingham retained his influence at court following the succession of Charles I, the damage to his reputation was enhanced by an equally disastrous assault on Saint-Martin-de-Ré in 1627 which St Leger also helped organise. Buckingham's assassination in August 1628 was greeted with widespread public rejoicing.^[6]

Ireland: 1627 to 1642

In March 1627, St Leger joined the Privy Council of Ireland and succeeded Buckingham's half-brother Sir Edward Villiers as President of Munster. Along with this position, he inherited Buckingham's struggle for control of the province with his local rival Richard Boyle, 1st Earl of Cork, and to counter this, St Leger became an ally of Thomas Wentworth when the latter was named Lord Deputy of Ireland in January 1632.^[7] In 1630, St Leger had proposed a new plantation scheme in County Tipperary, which was never implemented but aligned with Strafford's long-term policy of expanding Protestant cultural and religious dominance in Ireland.^[8]

However, in the short term, Strafford was less concerned with religious issues and more focused on increasing royal income by reclaiming lands that rightfully belonged to the Crown and ending widespread corruption among government office holders. Both particularly impacted so-called New English settlers like Boyle, who were the primary beneficiaries of such practices and viewed Catholicism as an immediate threat to their positions.^[7] When St Leger was elected Member of Parliament for County Cork in the Irish Parliament of 1634–1635 where he backed Strafford's policy of making limited concessions to the Catholic gentry in return for subsidies.^[9]

The first of the Bishops' Wars in 1639 ended with the Covenanters in control of Scotland, a defeat Charles I was determined to reverse. To help achieve this, in December 1639 Stafford called the 2nd Irish Parliament of King Charles I to approve funding for an Irish expeditionary force that would land in Western Scotland.^[10] St Leger was re-elected for County Cork to the Irish Parliament of 1640–1649.^{[lower-alpha 2][14]} Under Strafford's guidance, Parliament unanimously voted four subsidies of £45,000^[15] to fund an Irish army of 9,000 ^[16] for use against the Scots in the Second Bishops' War.^[17]

St Leger was appointed Sergeant Major^{[lower-alpha 3]} with responsibility for training the new recruits, ^[18] who were based in Carrickfergus ready for transport to Scotland or northern England. In July 1640, St Leger declared them ready for service but they remained in Ireland throughout the Second Bishops' War. The main impact was to deepen divisions between Strafford and his opponents in both the Irish and English Parliaments, which led to his trial and execution in May 1641.^[19]

An additional complication was what to do with the newly formed army, many of whom were Catholic and thus viewed with deep suspicion by the Protestant-dominated Dublin Castle administration.^[20] It was eventually agreed they could take service with Spain and when the Irish Rebellion began in October 1641, around 2,500 men under Garret Barry were waiting in Kinsale for shipment to the Army of Flanders.^[21]

As Lord President, St Leger was responsible for dealing with the insurgents in Munster, but the forces and supplies placed at his disposal were inadequate. In February 1942 he was reinforced by the arrival at Youghal of Charles Vavasour and his regiment.^[22] He also brought St Leger the royal declaration of 1 January 1642 in which the King denounced the rebels.^[23] St Leger was still struggling with the insurrection when he died on 2 July 1642, either at Doneraile Court^[24] or at Cork.^[25]